Boosted engines offer the performance of larger displacement engines with lower engine pumping work. However, at higher engine loads and lower engine speeds, combustion in an engine cylinder may be initiated automatically without inducing a spark. In particular, an air-fuel mixture may pre-ignite as a result of compressing a higher temperature air-fuel mixture. Pre-ignition of the air-fuel mixture can also further increase cylinder pressures so that cylinder end gases remaining in the cylinder automatically ignite, thereby causing higher levels of engine knock. The possibility of pre-ignition can be reduced or limited via introducing cooled exhaust gas residuals by way of an exhaust gas recirculation (EGR) system to the cylinder. Introducing cooled exhaust gas into a cylinder can slow chemical kinetics, thereby lowering the possibility of pre-ignition. In addition, cooled EGR disperses oxygen within the cylinder and reduces the rate of heat release of a combusted air-fuel mixture so that the possibility of igniting end gases is reduced. But, cooled EGR may not always be available to reduce the possibility of pre-ignition. For example, a sufficient amount of cooled EGR may not be available to limit the possibility of pre-ignition during transient conditions because it may be difficult to adjust the EGR amount to the transient conditions. Further, shortly after an engine start, EGR may not be available in the EGR system because of oxygen may have entered the exhaust system during an engine stop. Accordingly, EGR may be used to help control pre-ignition under some conditions, but it may not be capable of limiting the possibility of pre-ignition during all engine operating conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for controlling an engine, comprising: adjusting an engine torque limit in response to an amount of EGR in a cylinder of the engine.
The possibility of pre-ignition can be reduced by lowering the engine torque limit of the engine as the amount of EGR in the engine cylinders is reduced. For example, the engine torque limit can be reduced so that cylinder pressures in the engine have less possibility of reaching levels that facilitate pre-ignition when a position of a throttle is quickly adjusted. If the EGR percentage of charge entering engine cylinders cannot be adjusted at a rate that reduces the possibility of pre-ignition, the engine torque limit can be reduced so that cylinder pressures have less possibility of reaching pressures where pre-ignition is likely. Further, if EGR is requested and some air enters the engine intake system through the EGR system rather than EGR, the engine torque limit can be reduced until the air is purged from the EGR system and a full fraction of EGR is present in the cylinder charge. Once the air is purged from the EGR system, the engine torque limit can be increased as the percentage of EGR entering the engine cylinders increases.
The present description may provide several advantages. In particular, the approach can reduce the possibility of pre-ignition during transient conditions. Further, the approach can reduce the possibility of pre-ignition when less than a desired level of EGR is available in engine cylinders. Further still, the approach can be implemented in a variety of ways so that a particular torque actuator is not required to facilitate the approach.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.